Shortest path first restoration routing in a fiberoptic network

ABSTRACT

Interconnection failures in a communication network require the interconnections between the nodes of the network to be restored. SPF allows for the determination of the paths between the nodes in some optimal way. An improved implementation of SPF avoids or reduces the enormous amount of calculations required by SPF at each restoration operation.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This patent application claims priority from Provisional PatentApplication Nos. 60/215,399 and 60/215,182, both filed Jun. 29, 2000,and are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] The present patent application is related to fiberoptic networks,and, in particular, to switches for WDM and DWDM network systems.

[0003] In WDM (Wavelength Division Multiplexing) fiberoptic networks,optical signals are sent at predetermined wavelengths over opticalfibers. Each predetermined wavelength forms a communication channel inthe network and the wavelength (or frequency) of the optical signal isused to control the destination of the signal through the network. Anadvanced version of WDM networks is the DWDM (Dense Wavelength DivisionMultiplexing) network in which the number of wavelength channel isincreased by reducing the channel wavelength separation to 100 GHz, asset by the ITU (International Telecommunications Union). Hence the term,DWDM, is used herein to refer to both WDM and DWDM networks and otherfiberoptic networks which rely upon wavelength to define communicationchannels, unless indicated otherwise.

[0004] In networks, including such fiberoptic networks described above,switches or routers are used to select paths for signals through thenetworks. In fiberoptic networks switches and routers not only directoptical signals from one optical fiber to another but also from onewavelength channel to another. The availability of light paths iscritical to the users of a network. One way to provide reliability for alight path within the network is to explicitly provide for a redundantpath beforehand. However, this approach does not utilize the bandwidthof the network efficiently, i.e., some of the available network capacityis removed for the backup reserve.

[0005] The present invention, on the other hand, is directed towardon-the-fly light path restoration to achieve efficient bandwidth usageand availability at the same time. New paths are quickly reroutedthrough the network in place of the lost light paths. For the rerouting,the present invention provides for an efficient implementation ofso-called “Shortest Path First” (SPF) algorithm developed by F. W.Djikstra.

SUMMARY OF THE INVENTION

[0006] The present invention provides for a method of operation in acommunication network having a plurality of nodes interconnected bylinks between pairs of nodes, particularly a fiberoptic network. Themethod optimally selects paths between the nodes of the network. Eachsource node and a destination node forms a path of concatenated linksselected by an extreme value of a predetermined metric for the pathbetween the source node and the destination node after one or morefailed links of the paths. The method comprises maintaining for eachnode information of selected paths from the node to remaining nodes ofthe communication network in an order of the predetermined metric value;eliminating the information of each selected path having a failed linkin the path; selecting a path between the node and each one of theremaining nodes from the information of the determined paths providedthat the information of the path has not been eliminated; and selectinga path between the node and each one of the remaining nodes byconcatenating links having an extreme value of the predetermined metricfor the path if the information of the path has been eliminated.

[0007] The information includes a first database set of network pathsbetween nodes of the network, the paths ordered by the predeterminedmetric values; a second database set of evaluated network paths betweennodes of the network, the paths also ordered by the predetermined metricvalues; a database set of failed links; a database set of destinationnodes evaluated for a path from the node to each remaining node in thenetwork by the value of the predetermined metric, the database setincluding pointers from each destination node to the second database ofevaluated network paths so as to provide a selected path from the nodeto the destination node if the predetermined metric has an extremevalue.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is an exemplary DWDM network of a plurality of switch nodesoperating according to the present invention;

[0009]FIG. 2 is a flow chart of steps of conventional SPF operations forselecting paths in a network; and

[0010] FIGS. 3A-3C illustrate a flow chart of network restoration stepsaccording to the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0011]FIG. 1 illustrates an exemplary fiberoptic network with aplurality of switch nodes. Only five switch nodes 10-14 are shown forthe purposes of ease of explanation; more or less switch nodes could beused. Each of the switch nodes 10-14 is connected to external datafiberoptic lines 20, 22, 24, 26 and 28 respectively. For example, theswitch node 10 is connected to a plurality of fiberoptic lines connectedto sources and receivers (not shown) external to the fiberoptic network,which lines are represented by the line 20. Likewise, the switch node 11is connected to a plurality of externally-connected fiberoptic linesrepresented by the line 22, and so on. Within the fiberoptic network,the switch nodes 10-14 are interconnected by fiberoptic lines 21, 23,25, 27, 29, 30 and 31. The fiberoptic lines carry optical signals to,from, and between the nodes 10-14 at distinct and preselectedwavelengths which form network communication channels.

[0012] Details of the architecture and operation of the switch nodes10-14 are described in U.S. application Ser. No. 09/648,518, entitled,“Scalable DWDM Network Switch Architecture With Wavelength TunableSources,” filed Aug. 25, 2000 by Chien-Yu Kuo, Niraj Gupta and RonaldGarrison, which patent application is assigned to the present assigneeand which is hereby incorporated by reference. The memory andcomputational capabilities at each network for the distributedprovisioning and restoration operations of the network are disclosed inU.S. application Ser. No. ______, entitled “Method For Wavelength SwitchNetwork Restoration,” which patent application is also assigned to thepresent assignee and is hereby incorporated by reference. Provisioningrefers to the operations by which the nodes of a network receiveinformation to begin operations and restoration refer to operations.Both operations entail control and signaling operations by which thenetwork nodes are each provided information about the networkenvironment, e.g., the status of the other nodes and the paths alongwhich signals can pass from one node to another. The present inventionis concerned with the determination of these signal paths.

[0013] When a failure occurs in the connection between the nodes of thenetwork, the network connections must be restored. Various algorithmshave been proposed to make the “best” connections between the nodes of anetwork. The present invention uses an efficient implementation of the“Shortest Path First,” (sometimes termed SPF) developed by F. W.Djikstra. Descriptions of this algorithm can be found in many sources,such as the textbook, Routing in the Internet, Second Edition, byChristian Huitema, Prentice Hall, Upper Saddle River, N.J., p. 125. Innetwork terminology, the one-segment connection between two nodes is acommunication link. The connection between two nodes, which may be alink or a concatenation of links, is termed a path. It should be notedthat in the exemplary fiberoptic network illustrated in FIG. 1, thephysical link illustrated by a fiberoptic line between two nodes, sayline 21, is representative of many communication channel links. Incurrent fiberoptic networks, there are typically 64 or 65 wavelengthchannels on each fiberoptic line, most of which are used to carry databetween the nodes and some of which are used as substitute links when aregular communication link is faulty and the communication link isbroken. Hence the description below refers to communication links,unless explicitly stated otherwise.

[0014]FIG. 2 shows a flow chart of steps required by SPF to determinethe optimum connections after a network failure for one node S in thenetwork. After start step 40, database sets E and R are initialized sothat set E contains only the node S and set R contains all of theremaining nodes of the network. Then step 42 initializes a list O, anordered database of links, i.e., the one-segment paths, from the node S.In step 42 only the links from node S are placed in list O. By step 43,the list O (at this point containing only links from node S) is orderedby a predetermined metric. The particular order of the database is setby the “cost” of the path. Each link and, therefore, each concatenatedpath through the network, has an associated metric, or “cost.” Forexample, a metric might be the physical distance of the segments, thetransmission time, or the actual expense of transmitting messagesthrough the links. The list O provides that the links and paths areordered so that the “shortest” links and paths come first for the mostefficiency.

[0015] Decision step 44 asks if the list O is empty. If so, i.e., thereare no more paths from node S, the remaining nodes in set R is marked asunreachable from node S by step 46 and the process is terminated at step47. Even if the list O is not empty, similar decision step 45 checkswhether the first path in list O have an infinite metric, i.e., whetherthe “shortest” path in list O is open, indicating that there remains noreal path from node S. If the metric is infinite, step 46 marks theremaining nodes in set R as unreachable and the process is terminated.

[0016] If the metric is not infinite, then step 48 removes the shortestpath P from list O. Step 49 checks whether the last node, termed V, inthe removed path P is already in set E. In other words, has a path fromnode S to node V already been found? If yes, the process is returnedback to step 44. If not, the node V is moved from set R (of remainingnodes) to set E (of evaluated nodes) by step 50. In step 51, new pathsfrom node S to nodes connected to node V by links are determined byconcatenating path P to each of the links connected to node V. The newpaths are placed into, and ordered in, list O by step 52. The cost ofthe new paths is the sum of the cost of path P and the metric of eachlink from node V to the nodes connected to node V.

[0017] The process continues until all the paths from node S aredetermined. In the case of the exemplary fiberoptic network of FIG. 1,the process continues for all the wavelength channels of the network.This SPF process is invoked every time a new failure, such as a linkgoing down, in the network occurs and new paths must be calculated foreach node in the network. It should be noted that with any network ofconsequence, the calculations required to determine the optimum paths bySPF are enormous. For example, with a network of 100 nodes and eachoptical fiber link carrying 64 wavelength channels, the break in onefiber link requires the calculations described above, a very largenumber of calculations indeed.

[0018] In contrast, the present invention reduces the calculationsrequired by SPF to determine the optimal paths in a network restoredafter a failure. Hence the speed of restoration of the connections aftera failure is greatly enhanced.

[0019] For each node of the network, the present invention storesinformation about the links and connecting paths of the network channelfrom that node for each wavelength. Some of the databases are the sameas described above, others are expanded with more information, whilestill others contain different. The databases include the list O ofordered paths, a list Q of ordered paths which have been alreadyconsidered; the database set E of evaluated nodes and a database set Uof links unavailable since the last calculation of paths forinterconnecting the network nodes. The database set U of unavailablelinks permits some flexibility in the path restoration operations, whichmust be performed immediately if the database set U did not exist. Eachof the nodes of the database set E contains information includesidentification of the destination node from the source node along apath, a list L of pointers to the paths in list Q with each pointerassociated with a destination node. Paths are stored with information oftheir source node, their destination node and the set PL of orderedlinks from the source node to the destination node.

[0020] Upon invocation of the process, the determination of theconnecting paths are carried as follows according to the presentinvention. FIGS. 3A-3C illustrate a flow chart of steps requiredaccording to the present invention. After start step 60, decision step61 asks if the databases sets E and U, and lists O and Q, as describedabove exist, are nonempty, or in other words, if a routing procedure hasbeen performed previously. If no, then the process moves to step 71 onFIG. 3B. If yes, then the process eliminates paths and pointers to thepaths with links which are now unavailable. Step 62 deletes paths whichhave links now unavailable (links in database set U) from list Q andcorresponding pointers in list L to these paths in list Q. Step 63likewise deletes paths which have unavailable links from list O and step64 then reinitializes the database set U to empty. For any remainingdestination nodes in database set E, step 65 follows the correspondingpath in list Q as indicated by the corresponding pointer in list L,i.e., a connection has been found from the source node to thedestination node.

[0021] The process then continues to the operations described in FIGS.3B and 3C, which have many steps similar to the those describedpreviously with respect to the conventional SPF operations. By step 71database sets E and R, augmented as described above, are initialized sothat set E contains only the source node S and set R contains all of theremaining nodes of the network. Then step 72 initializes the list 0, theordered database of links, i.e., one-segment paths, from the node S. Bystep 73, the list 0 (at this point containing only links from node S) isordered by the predetermined metric. As described previously, the list 0provides that the links and paths are ordered so that the “shortest”links and paths come first.

[0022] Decision step 74 asks if the list O is empty. If so, i.e., thereare no more paths from node S, the remaining nodes in set R is marked asunreachable from node S by step 76 and the process is terminated at step77. Even if the list O is not empty, similar decision step 75 checkswhether the first path in list O have an infinite metric, i.e., whetherthe “shortest” path in list O is open, indicating that there remains noreal path from node S. If the metric is infinite, step 76 marks theremaining nodes in set R as unreachable and the process is terminated.

[0023] If the metric is not infinite, decision step 83 determineswhether the last link in the path under consideration is a substitutelink which exhausts the spare capacity of the link. The optical fiberbetween the two nodes has no more spare channels available to substitutefor nonfunctioning channels. If not, the process continues to step 78 tobuild up the network paths from the source node S. If so, step 84 placesthe exhausted link in list U of unavailable links and the information isbroadcast to the other network nodes. Then the process returns to step78 and the process of finding the network paths from source node Scontinues. The shortest path P is removed from list O and step 79 checkswhether the last node, termed V, in the removed path P is already in setE. In other words, has a path from node S to node V already been found?If yes, the process returns back to step 74. If not, the node V is movedfrom set R (of remaining nodes) to set E (of evaluated nodes) by step80. In step 81, new paths from node S to nodes connected to node V bylinks are determined by concatenating path P to each of the linksconnected to node V. The new paths are placed into, and ordered in, listO by step 82. The cost of the new paths is the sum of the cost of path Pand the metric of each link from node V to the nodes connected to nodeV.

[0024] The process loops back to step 73 and continues until all thepaths from node S are determined.

[0025] The present invention reduces the amount of network restorationcalculations tremendously for networks of any size. If M equals thenumber of physical links, i.e., optical fibers between nodes, and C isthe number of circuits of a failed link, i.e., the number of connectionpaths using the failed link, in a network, conventional SPF calculationrequire an order of calculations of C*M*(log M). On the other hand,network restoration calculations according to the present invention asdescribed above are an order of(N−1)*M*(log M) calculations with N beingthe number of nodes in the network. As the complexity of the networkincreases, it is evident that the restoration calculations in accordancewith the present invention become increasingly more efficient.

[0026] Further improvements to the network restoration are possible. Forexample, in the determination of the network paths, there is an implicitassumption that the communication paths are unidirectional.Communication is from source node S to a destination node D. In thefiberoptic network described, communication is bidirectional so that acommunication path once determined from node S to node D can be reversedso that a communication path from node D to node S is also determined.This creates another savings in network restoration calculations.

[0027] Therefore, while the description above provides a full andcomplete disclosure of the preferred embodiments of the presentinvention, various modifications, alternate constructions, andequivalents will be obvious to those with skill in the art. For example,while the present invention has been described in terms of fiberopticnetworks, it should be applicable to other types of networks. Thus, thescope of the present invention is limited solely by the metes and boundsof the appended claims.

What is claimed is:
 1. A method of selecting paths between nodes of acommunication network, each path between a source node and a destinationnode formed by concatenated links selected by an extreme value of apredetermined metric for said path between said source node and saiddestination node after one or more failed links of said paths, saidmethod comprising maintaining for each node information of selectedpaths from said node to remaining nodes of said communication network inan order of said value of said predetermined metric; eliminating saidinformation of each selected path having a failed link in said path;selecting a path between said node and each one of said remaining nodesfrom said information of said determined paths provided that saidinformation of said path has not been eliminated; and selecting a pathbetween said node and each one of said remaining nodes by concatenatinglinks having an extreme value of said predetermined metric for said pathif said information of said path has been eliminated.
 2. The method ofclaim 1 wherein said information comprises a first database set ofnetwork paths between nodes of said network, said paths ordered byvalues of said predetermined metric; a second database set of evaluatednetwork paths between nodes of said network, said paths ordered byvalues of said predetermined metric; a database set of failed links; adatabase set of destination nodes evaluated for a path from said node toeach remaining node in said network by said value of said predeterminedmetric, said database set including pointers from each destination nodeto said second database of evaluated network paths so as to provide aselected path from said node to said destination node if saidpredetermined metric has an extreme value.
 3. The method of claim 2further comprising setting said database set of failed links to emptyafter said eliminating step.
 4. In a communication network having aplurality of nodes interconnected by links between pairs of said nodes,a method of selecting paths between each node and a destination nodeformed by concatenated links selected by an extreme value of apredetermined metric for said path between said source node and saiddestination node after one or more failed links of said paths, saidmethod comprising maintaining for each node information of selectedpaths from said node to remaining nodes of said communication network inan order of said value of said predetermined metric; eliminating saidinformation of each selected path having a failed link in said path;selecting a path between said node and each one of said remaining nodesfrom said information of said determined paths provided that saidinformation of said path has not been eliminated; and selecting a pathbetween said node and each one of said remaining nodes by concatenatinglinks having an extreme value of said predetermined metric for said pathif said information of said path has been eliminated.
 5. The method ofclaim 4 wherein said communication network comprises a fiberopticnetwork, each link comprising a wavelength channel between a pair ofnodes.
 6. The method of claim 5 wherein said information comprises afirst database set of network paths between nodes of said network, saidpaths ordered by values of said predetermined metric; a second databaseset of evaluated network paths between nodes of said network, said pathsordered by values of said predetermined metric; a database set of failedlinks; a database set of destination nodes evaluated for a path fromsaid node to each remaining node in said network by said value of saidpredetermined metric, said database set including pointers from eachdestination node to said second database of evaluated network paths soas to provide a selected path from said node to said destination node ifsaid predetermined metric has an extreme value.
 7. The method of claim 2further comprising setting said database set of failed links to emptyafter said eliminating step.